Study sheet: Cell Cycle and Cell Division

📋 Course Outline

1. Cell Cycle Organization
2. Interphase and Quiescence
3. Mitosis and Nuclear Division
4. Cytokinesis Mechanisms
5. Biological Significance of Mitosis
6. Meiosis I and Recombination
7. Meiosis II and Its Significance

📖 1. Cell Cycle Organization

🔑 Key Concepts & Definitions

• Cell cycle : The sequence of events through which a cell duplicates its genome, synthesises other cellular constituents, and divides into two daughter cells.

★ Must-know

📌 The cell cycle has two basic phases: interphase, which prepares the cell for division, and M phase, during which mitosis and usually cytokinesis occur.

Further detail

• A typical human cell in culture completes one cell cycle in approximately 24 hours, whereas yeast can complete one in about 90 minutes.

💡 Memory Hook

G1 growth → S synthesis → G2 preparation → M division

📖 2. Interphase and Quiescence

🔑 Key Concepts & Definitions

• Quiescent stage : An inactive cell-cycle stage in which cells remain metabolically active but no longer proliferate unless required by the organism.

📝 Essential Points

• Interphase consists of G1, S, and G2 phases, during which the cell grows, replicates its DNA, and synthesises proteins in preparation for mitosis.

• During S phase, DNA content increases from 2C to 4C, but the chromosome number remains unchanged; a diploid cell remains 2n.

💡 Memory Hook

G1 grows without DNA replication, S replicates DNA, G2 prepares for mitosis

📖 3. Mitosis and Nuclear Division

★ Must-know

• 🔄 Mitosis proceeds through four stages:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

• During prophase, chromatin condenses into chromosomes composed of two sister chromatids, centrosomes move toward opposite poles, and the mitotic spindle forms.

• During metaphase, spindle fibres attach to kinetochores and align the chromosomes at the equator on the metaphase plate.

• During anaphase, centromeres split and sister chromatids separate and migrate to opposite poles as daughter chromosomes.

Further detail

• During telophase, chromosomes decondense, nuclear envelopes form around the chromosome groups, and the nucleolus, Golgi complex, and endoplasmic reticulum re-form.

💡 Memory Hook

Prophase → Metaphase → Anaphase → Telophase

📖 4. Cytokinesis Mechanisms

★ Must-know

📌 Karyokinesis is the division of the nucleus, whereas cytokinesis is the division of the cytoplasm that completes cell division.

📌 Animal cytokinesis occurs by an inward-cleaving plasma-membrane furrow, whereas plant cytokinesis begins with a central cell plate that grows outward to the lateral walls.

Further detail

• During cytokinesis, mitochondria and plastids are distributed between the two daughter cells.

💡 Memory Hook

Animal cells pinch inward; plant cells build a plate outward

📖 5. Biological Significance of Mitosis

🔑 Key Concepts & Definitions

• Equational division : Mitosis is called equational division because the chromosome number is conserved in the daughter cells.

📝 Essential Points

📌 Mitosis usually produces diploid daughter cells with chromosome complements genetically identical to the parent cell.

• Mitosis contributes to: growth of multicellular organisms, cell repair, replacement of epidermal cells, replacement of gut-lining cells, replacement of blood cells, lifelong plant growth in apical and lateral cambium meristems

💡 Memory Hook

Mitosis preserves chromosome number → growth, repair, and replacement

📖 6. Meiosis I and Recombination

🔑 Key Concepts & Definitions

• Meiosis : A specialised division of diploid cells that reduces the chromosome number by half and produces haploid daughter cells for sexual reproduction.

★ Must-know

• Meiosis consists of two successive nuclear and cell divisions, meiosis I and meiosis II, following a single cycle of DNA replication.

• During prophase I, homologous chromosomes pair by synapsis, form bivalents, exchange genetic material between non-sister chromatids by crossing over, and become linked at chiasmata.

• In anaphase I, homologous chromosomes separate and move to opposite poles while sister chromatids remain joined at their centromeres.

Further detail

• The five stages of prophase I are:
  • leptotene
  • zygotene
  • pachytene
  • diplotene
  • diakinesis

💡 Memory Hook

Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis

📖 7. Meiosis II and Its Significance

🔑 Key Concepts & Definitions

• Interkinesis : The generally short stage between meiosis I and meiosis II during which DNA is not replicated.

★ Must-know

• Meiosis II resembles mitosis: chromosomes align at the equator, sister chromatids separate after centromere splitting, and nuclei reform during telophase II.

• At the end of meiosis II, cytokinesis produces a tetrad of four haploid daughter cells.

• Meiosis preserves the species-specific chromosome number across sexual generations because fertilisation restores the diploid phase after meiosis produces haploid gametes.

Further detail

• Meiosis increases genetic variability between generations, and this variation is important for evolution.

💡 Memory Hook

Two divisions after one replication → four haploid cells and genetic variation

📊 Synthesis Tables

Mitosis and Meiosis Compared

Feature	Mitosis	Meiosis
Number of divisions	One	Two successive divisions
DNA replication	Before the division	Once before meiosis I
Chromosome outcome	Usually conserved	Reduced by half
Products	Usually two genetically identical daughter cells	Four haploid daughter cells
Genetic variation	Usually not increased	Increased through recombination and chromosome segregation